Can you use a torque wrench to inspect a bolt for proper torque?

 What happens when you tighten a bolt? The bolt is pulled longer, “stretched”, and being springy it wants to return to its original length by pulling the joint together. The joint surfaces push back at the bolt keeping the bolt stretched. The bolt is strained in tension; the joint in compression; and our parts are firmly clamped together.

But how did we get the bolt to stretch? We tightened it, not only by stretching along its axis, but by twisting it. We stretched AND twisted the bolt. The stretch is necessary as it keeps our surfaces clamped together, but what is the twisting force doing? It might have gone into loosening our nut slightly the moment we released tension on our wrench; or it might still be locked into the joint in which case all of the wrench force you used to turn the nut may still be there trying to loosen the nut. It all depends on the friction between the nut face and the seating surface (an important lubrication consideration).

What happens if we use a torque wrench to “check the torque” on a bolt? If we torque the nut until we reach the “breakaway torque” we have to apply enough torque to overcome the friction between the nut face and joint surface. Until this happens none of our torque is felt by the bolt. Once the nut face is released, torque starts to twist the bolt until it overcomes the friction between the male and female threads. Only now does the nut move relative to the bolt and we detect the “breakaway” torque.

Notice that the only thing we are measuring is friction. Everything depends on friction. If the friction is more or less than when the bolt was originally tightened then our “breakaway” torque will be more or less than the torque that the assembler applied to the nut. That is the quandary.  Did the assembler improperly torque the nut or did the friction change? We have no way of knowing. 

“The nuts were loose.” Can we check assembly torque by loosening the nut?  Same problem but only worse because if the twisting force is locked into our bolt then it might take only the smallest amount of torque to get the nut moving (our breakaway torque) as the bolt tries to unwind. 

But what if the nut really is loose? Did the mechanic not tighten the nut properly? We still don't know. Take your pick; all or some of the reasons the nut might be loose from the list: 

  • embedment relaxation of the faying surfaces
  • bolt stretch from metal creep (especially at high temperature)
  • nut backed-off  due to vibration loosening
  • wasn’t tightened properly to begin with
  • elastic interactions between multiple bolts in a flange has reduced preload (crosstalk).

Determining the degree of tightness in a joint by using a torque wrench to measure the breakaway torque is not accurate and leads to incorrect conclusions.  Beware of inspectors carrying torque wrenches.

 

Comments

I think the answer depends on the actual question you’re asking.

I have had this discussion with mechanics, to which I have found much confusion.

In accordance with Bell Helicopter Standard Practice Manual, checking fasteners accurately to determine if they have been tightened to the required Assembly Torque is not possible. When there is doubt as to whether a fastener has been tightened to the correct torque value, the fastener should be backed off from one-half to one full turn and retightened to the correct toque value.

However, there are maintenance procedures that require "Re-Torques" and "Torque Checks". Re-torque: when a re-torque is specified in the applicable maintenance publication or it is uncertain if a joint has been properly torqued, back off the fastener on or two turns, then tighten to the required torque as follows:

- NOTE: DO NOT USE TORQUE WRENCH TO LOOSEN FASTENERS –
a. Remove all torque from the fastener (loosening) until no
preload is on the fastener.

b. Measure the tare torque.

c. Determine the specified torque or standard torque, as
applicable, and add the measured tare torque to determine
the Assembly Torque.

d. Torque the fastener to this Assembly Torque value.

e. If the fastener is one in a multi-fastener patter requiring a
torque sequence, then all the fasteners will require "re-
torque."

Torque Check: When a Torque Check is called out in the applicable maintenance publication, it should be accomplished by torquing in the tightening direction. Do not loosen the fastener(s). The value applied should be one of the following:

a. If the Assembly Torque is known from when the fastener was
originally installed (i.e., recorded in a logbook), then this is the
Assembly Torque for Torque Check purposes.

b. If the original Assembly Torque was not recorded, then the
Assembly Torque for Torque Check purposes would be the
minimum Specified Torque or minimum Standard Torque, as
applicable, plus the minimum acceptable Tare Torque.

c. If during the application of the Assembly Torque as detailed
above, no motion is detected between the fasteners, then the
joint is considered acceptable.

d. Joints which are having a Torque Check performed as part of a
Special Inspection, as required after a specified number of
flight hours, only need to be tightened. Looseness may occur
until the components “seat” themselves and the fasteners
simply need to be tightened. This is not cause for disassembly;
however, the fastener(s) will have to be Torque Checked
again at the same scheduled interval set for the first Torque
Check until the assembly is completely seated. If a specific
torque sequence is to be followed, as initially torque, then this
same sequence should be followed during the Torque Check.
Some fasteners in the sequence may accept additional torque
while others may not; this is acceptable.

e. Joints that have not retained torque will require disassembly
and inspection. If the fastener(s) move, the assembly shall be
rechecked for damage, corrosion, improper assembly, etc.; if
no problem is found, the fastener(s) may be re-torqued to the
Assembly Torque value. In this case, the fastener(s) would
have to be Torque Checked again at the same scheduled
interval set for the first Torque Check.

Thank you for posting this intriguing question. I guess it's one of my pet peeves that some mechanics take torque for granted and don't investigate the true nature of torque checks and re-torques. Theres a difference for a reason and the procedure outcomes can be concern for further investigation.

By Rena H. Smith, FAA A&P with I.A - Technical Inspector

Thanks for the insight on retorquing fastners. I think your procedure spelled out here, is pretty much a standard, most likely industry-wide. I remember a very similar procedure used by the then McDonnell Douglas Aircraft in Long Beach. In maintenance, if the fastner is found loose, running it all the way out is a good check to see if it has broken off somewhere. Sometimes you might get a good torque value on a bolt, but it could still be damaged. That's particularily for bolts that are bilnd-sided.

Again, thank you very much.

Vic

Great answer Rena. I used to catch mechanics doing a retorque on a bolt that was safety wired to prevent loosening. They would slip the socket on as far as they could, let torque wrench click as they scar up the safety wire, then try and cover up the damaged original safety wire with a blob of torque stripe paint. It used to boggle their minds when I would explain the need to cut the safety wire and back it up a turn.

Posted by Daniel Loomis

The last line in this article, "beware of inspectors carrying torque wrenches", inspectors and mechanics alike should know that you can use a dial type torque wrench (which have better accuracy) to measure "tare torque" on a threaded fastener combination. AC43-13 has a table for the type of fastener vs. they type of application as well (shear vs. tension). We can all use a little refresher every now and then.

Thanks for the insight on retorquing fastners. I think your procedure spelled out here, is pretty much a standard, most likely industry-wide. I remember a very similar procedure used by the then McDonnell Douglas Aircraft in Long Beach. In maintenance, if the fastner is found loose, running it all the way out is a good check to see if it has broken off somewhere. Sometimes you might get a good torque value on a bolt, but it could still be damaged. That's particularily for bolts that are bilnd-sided.

Again, thank you very much.

Vic

By Victor Cook, C-130 Maintenance Technician at Boeing Internation Support Systems via LinkedIn

Great comments. I'll add, if it's loose, replace it if it is structural and find out why it was loose. Often leads to other stories... Thanks for the comments.

Posted by Bill Archer via LinkedIn